As a subclass of commercially available polymers, polyurethane elastomers have several properties whose advantages confer unique benefits on these products. Typically, polyurethanes show high abrasion resistance with high load bearing, excellent cut and tear resistance, high hardness, resistance to ozone degradation, yet are pourable and castable. Compared to metals, polyurethanes are lighter in weight, less noisy in use, show better wear and excellent corrosion resistance while being capable of less expensive fabrication. Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibita good elastomeric memory. Polyurethanes find use in such diverse products as aircraft hitches, bushings, cams, gaskets, gravure rolls, star wheels, washers, scraper blades, impellers, gears, and drive wheels.
Part of the utility of polyurethanes derives from their enormous diversity of properties resulting from a relatively limited number of reactants. Typically, polyurethanes are prepared on site by curing urethane prepolymers, which are adducts of polyisocyanates and polyhydric alcohols. A large class of such prepolymers are approximately 2:1 adducts of a diisocyanate, OCN--Y--NCO, and a diol, HO--Z--OH, whose resulting structure is OCN--Y--NHCO.sub.2 --Z--OC(O)NH--Y--NCO. Although Y is susceptible of great variety, usually being a divalent alkyl, cyclohexyl, or aromatic radical, in fact the most available prepolymers are made from toluene diisocyanate (TDI), most readily available as a mixture of 2,4- and 2,6-isomers which is rich in the former isomer, or methylene-4,4'-diphenyldiisocyanate (MDI). The diols used display a greater range of variety; Z may be a divalent alkyl radical (i.e., an alkylene group), and the diols frequently are ethers or esters which are the condensation products of glycols with alkylene oxides or dicarboxylic acids, resp.
The polyurethane elastomers are formed by curing the prepolymer. Curing is the reaction of the terminal isocyanate groups of the prepolymer with active hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, crosslinking. Diols, especially alkylene diols, are the most common curing agents for MDI-based prepolymers, and representing such diols with the structure HO--X--OH, where X is an organic moiety, most usually an alkylene group, the resulting polymer has as its repeating unit, EQU (--Y--NHCO.sub.2 ZOC(O)NH--Y--NHCO.sub.2 --X--O--C(O)NH--).
Where a triol or a higher polyhydric alcohol is used crosslinking occurs to afford a nonlinear polymer.
Although other polyfunctional chemicals, especially diamines, are theoretically suitable, with but a few exceptions none have achieved commercial importance as a curing agent. The major exception is 4,4'-methylene-di-ortho-chloroaniline, usually referred to as MOCA, a curing agent which is both a chain extender and a crosslinker. TDI-based prepolymers typically are cured with MOCA, and the resulting products account for perhaps most of the polyurethane elastomer market. One reason that polyhydric alcohols generally have gained acceptance as curing agents is that their reaction with urethane prepolymers is sufficiently fast to be convenient, but not so fast as to make it difficult to work with the resulting polymer. In casting polymers it is desirable that the set-up time be reasonably short, yet long enough for the material to be cast into molds. This property is conventionally referred to as pot life. Generally speaking, diamines react with prepolymers, and especially MDI-based prepolymers, so quickly that they are not usable as curing agents. However, primary aromatic diamines with electronegative groups on the aromatic ring, or with alkyl groups ortho to the amino moiety, exhibit sufficiently decreased reactivities with some prepolymers as to afford a desirable pot life, hence the use of, for example, MOCA as a curing agent for TDI-based prepolymers. However, MOCA and others of the aforementioned diamines still remain too reactive to be used, for example, with MDI-based prepolymers.
On the other hand, the advent of reaction injection molding (RIM) provides a means of processing polyurethanes which is well adapted to a short pot life. Reaction injection molding is a process that allows polymerization and crosslinking to take place simultaneous with forming of a part into its final shape. Because of the rapid curing of polyurethanes, compatible with the fast cycle times of RIM, these polymers seem exceptionally well suited to RIM processing although epoxies, nylons, and even polyesters have been made by the RIM process.
In RIM, two highly reactive streams of chemicals are brought together under high pressure in a small mixing chamber where the streams are impingement mixed by being sprayed directly into each other before entering the mold. The mixed material flows directly into a mold at 0.35-0.7 MPa (50-100 psi), a low pressure compared to that used in standard injection molding, where the chemical reaction is completed and the part cures. One of the ingredient streams (the first stream) contains the isocyanate and the other stream (the second stream) contains components having isocyanate-reactive hydrogens, such as polyols and amines, and other components as catalysts, pigments, blowing agents, and surfactants. Much of the technology is currently used in the automotive industry to produce parts such as bumper covers and fenders. Parts are produced on a cycle of 3 minutes or less, and large urethane parts have been successfully demolded in 30 seconds or less after injection.
We have found that a large class of N,N'-disubstituted aromatic diamines, mainly alkylated methylene dianilines, are excellent isocyanate-reactive components, or curing agents, for polyisocyanates in the preparation of RIM elastomers. Among the advantages of the curing agents of this invention are that the resulting elastomers can be expected to show excellent compression set, to have quite high tensile strength, show greater elongation, and increased impact properties. The resulting elastomers are thermoplastic or thermosetting polymers, depending on whether or not a crosslinking agent has been added, or if any of the other components present are capable of crosslinking. Additionally, the curing agents themselves for the most part are liquids at room temperature, facilitating their use at RIM temperature. The curing agents may be used for both TDI and MDI-based polyisocyanates, which give rise to the two largest classes of polyurethane and polyurea elastomers. In short, the unique properties of both the curing agents and the resulting elastomers make each very highly desirable in RIM formulations.
The curing agents of this invention are secondary aromatic diamines, which as a class formerly were not considered as acceptable curing agents for polyurethane and polyurea elastomers. U.S. Pat. No. 3,846,351 describes the quite narrow use of secondary aromatic alkyl diamines in combination with polyols as catalysts and chain extenders in the production of flexible polyurethane foams. It is important to recognize that such secondary diamines were used only in combination with polyols, preferably at the relatively low level of 0.5-5 parts per 100 parts of polyol. In an examination of aromatic diamines as chain extenders in polyurethane elastomers by reaction injection molding, the sole secondary aromatic diamine examined, N,N'-diisopropyl-4,4'-methylenedianiline, afforded products whose properties were generally unacceptably inferior to those exhibited by polyurethanes made with primary aromatic diamines as curing agents. "D. Nissen and R. A. Markovs," Proceedings of the 27th SPI Annual Technical/Marketing conference, 71-8. More recently we have shown in U.S. Pat. No. 4,578,446 that contrary to the prior conventional wisdom alkylated methylenedianilines are suitable curing agents for urethane prepolymers, i.e., in elastomer production via non-RIM processes.
To aid in exposition the isocyanate-reactive components can be classified as either polyols (polyhydric alcohols) or polyamines. Each of these classes has two functionally defined subclasses; backbone polyols (or polyamines) and chain extender polyols (or polyamines). The difference is that, e.g., the backbone polyol, being more reactive, reacts with the isocyanates in the first stream to afford short polymeric segments, and the chain extender polyamine links the short segments to form longer chains. The diamines of this invention act as chain extender polyamines.
The RIM elastomers which can be made from the amines of this invention are diverse and depend upon the nature of the isocyanate-reactive stream. In one variant the second stream as the isocyanate-reactive component is a mixture of backbone and chain extender polyamines. That is, the second stream may have catalysts, pigments, surfactants, etc., but contain little, if any, isocyanate-reactive components other than the polyamines. In this variant the elastomer is exclusively, or almost so, a polyurea.
In another variant the second stream has as the isocyanate-reactive component a mixture of the amines of this invention, which act as chain extender amines, and various backbone polyols. The mixture will generally have from about 20 to about 80% of amine, on an equivalents basis, and more usually contains 30-70 equivalents percent of amine. (An equivalent of polyamine or polyol is an amount which furnishes as many amino or hydroxyl groups as there are isocyanate groups in the first stream. As used herein, "equivalents percent" refers to the percentage of amine and/or polyol equivalents relative to isocyanate equivalents.)
In each of the foregoing variants a portion of the chain-extender diamines of this invention may be replaced by a chain extender polyol or a second chain extender polyamine. Although this will be described more fully within, to exemplify one of these subvariants the second stream may contain a backbone polyamine, the chain extender diamines of this invention, and a chain extender polyol where the polyol level is roughly 20-100 equivalents percent of the chain extender diamine.